Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials. The junction of the n-type window layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type absorber layer (i.e., an electron accepting layer) and the CdS layer acts as a n-type window layer (i.e., an electron donating layer). Similarly, photovoltaic cells can be made that include copper indium gallium (di)selenide (CIGS) as the p-type layer.
However, CdS in the n-type window layer generally does not generate any current in a CdTe-based or CIGS-based solar cells due to its extremely short minority carrier lifetime. Since CdS (having an optical bandgap of about gap 2.42 eV) absorbs in the blue region of the spectrum, a thick layer of CdS lowers the current produced by the solar cell. As such, obtaining very high currents with a p-type layer of CdTe requires a relatively thin layer of CdS, which generally leads to low voltage. At the same time, a minimum thickness of CdS is needed to form a robust junction and generate high open circuit voltage. Accordingly, the CdS thickness is in balance.
Thus, a need exists for increasing solar cell efficiency by simultaneously attaining high voltage and high current through changing the dynamic of the n-type window layer.